1. Field of the Invention
The present invention relates to a tread rubber composition improved in grip performance on wet road surfaces, especially on ice-covered and snow-covered road surfaces, and in abrasion resistance, and a pneumatic tire employing the same.
2. Description of the Background Art
In recent years, in response to the demands for fuel-efficient automobiles, tires decreased in rolling resistance and improved in wet grip performance have been developed. As a technique to decrease the rolling resistance, carbon black conventionally used as a reinforcing agent has been partially replaced with silica, in an effort to balance the antinomic properties of fuel efficiency and grip performance.
For example, U.S. Pat. No. 5,227,425 discloses a technique to improve the grip performance by a combination of specific silica and mixing method.
Japanese Patent Laying-Open No. 2-135241 discloses a technique to improve ice skid properties, wherein 100 parts by weight of natural rubber and/or diene-based synthetic rubber is blended with 10–80 parts by weight of low-temperature plasticizer having a freezing point of not greater than −48° C. and 5–40 parts by weight of silicon carbide, silicon nitride or the like having an average particle diameter of 0.1–1 mm.
Further, Japanese Patent Laying-Open No. 60-147450 discloses a tire tread rubber composition having high coefficient of friction on ice, wherein 100 parts by weight of natural rubber and/or diene-based synthetic rubber is blended with 10–80 parts by weight of low-temperature plasticizer having a freezing point of not greater than −40° C. and 5–45 parts by weight of alumina having an average particle diameter of 0.01–0.5 mm.
Still further, Japanese Patent Laying-Open No. 2001-181447 discloses a tread rubber composition improved in both grip performance and abrasion resistance and reduced in rolling resistance, wherein 100 parts by weight of rubber component including at least 60 parts by weight of styrene-butadiene copolymer containing 20–60% by weight of styrene is blended with 50–80 parts by weight of aluminum hydroxide having an untamped density of not greater than 0.6 g/cm2, a DOP oil absorption of not lower than 70 cm3/100 g and less than 250 cm3/100 g and a BET specific surface area of 30–350 m2/g, and 10–100 parts by weight of carbon black having a BET specific surface area of not lower than 70 m2/g.
The conventional techniques described above, however, fail to improve abrasion resistance and grip performance both at ordinary temperature (about 20° C.) and at low temperature (about 0° C.). A tread rubber composition which can keep its properties at low temperature the same as at ordinary temperature, has conventionally been demanded.